1. Technical Field
This invention relates generally to free-piston type internal combustion engines, compressors or pumps, and in particular, to innovations which improve the controllability and efficiency of the free-piston engine or pump and reduce the toxic emissions, the weight and the size of such engines.
2. Background Art
Although advantageous in applications where pressurized fluid is used to transmit the energy, the simple concept of free-piston internal combustion engines or pumps, transferring the chemical energy of a combustible fuel direct into mechanical energy of pressurized hydraulic fluid, is rarely utilized due to the inability to control their operating characteristics, and in particular, the top and bottom end positions of the piston, sufficiently.
Free-piston Control
In one known free-piston engine, disclosed in U.S. Pat. No. 4,791,786, the hydraulic piston control mechanism of the free-piston requires three hydraulic control surfaces in axial direction to control the top-end position and the bottom-end position somewhat sufficiently. The free-piston has a combustion end and a hydraulic end, consisting of a plunger with one outer control surface acting in opposite direction to the combustion forces, and a piston with one larger control surface acting also in the opposite direction and a smaller control surface acting in the same direction as the combustion force. During the compression stroke, pressurized fluid at the outer plunger surface advances the free-piston toward the top-end position while the chamber at the larger piston surface draws fluid in from a reservoir. The smaller piston surface, acting in opposite direction and being smaller than the plunger surface, is permanently pressurized with hydraulic fluid and provides a buffer function at the end of the compression stroke in top-end position by depressurizing the plunger surface. During the expansion stroke, all three control surfaces are exposed to pressurized fluid, advancing the fluid, drawn in during the compression stroke, to the accumulator. The bottom-end position of the free-piston can be obtained by readjusting the position after a stop.
In this prior art free-piston engine, the top-end position is determined by the balance of the buffer force, depending on the fluid pressure in the accumulator, and the dynamic mass forces of the heavy free-piston, depending on its velocity. The bottom-end position is determined by the balance of combustion and dynamic piston mass forces versus hydraulic forces, and depends on the insufficiently controllable, variable velocity of the piston (rpm.) and the fluid pressure in the accumulator. During a cycle interruption, the bottom-end position can be corrected.
The variations in top-end position and bottom-end position are too high to allow for an overall sufficient control of the compression ratio and combustion conditions, reducing the efficiency and increasing the amount of toxic emissions. Furthermore, the requirement for three hydraulic control surfaces increases the cost and size of the free piston engine and reduces the efficiency of the free-piston engine.
In another known free-piston engine, which is disclosed in U.S. Pat. No. 5,556,262, the hydraulic piston control mechanism consists of four control surfaces in axial direction to control the top-end position and the bottom-end position. The free-piston has a hydraulic end, consisting of a compression section, having a larger and a smaller control surface, and a pump section, also having a larger and a smaller control surface in which the larger surfaces are acting in opposite direction to the combustion end of the free-piston assembly.
During the compression stroke, the smaller control surface of the compression section is in communication with the fluid reservoir and the larger control surface is pressurized with fluid from a compression (bouncing) accumulator, advancing the free-piston toward the top-end position, while the pump section of the hydraulic end draws fluid in from the reservoir. During the expansion stroke, the hydraulic section, controlled by non-return valves, advances the fluid to the pressure accumulator, while the pressure conditions in the compression section remain unchanged. The bottom-end position is determined by decreasing combustion forces and increasing hydraulic forces. The hydraulic section has no noticeable influence in the control of the free-piston.
The top-end position is determined by the balance of nearly constant hydraulic forces and mass forces of the free-piston, varying with the velocity of the free-piston, and the compression forces acting in opposite direction. The bottom-end position is controlled by the mass forces of the free-piston in addition to the combustion pressure and the increasing hydraulic forces. Increased accuracy of the bottom-end position is obtained with increasing hydraulic losses to brake the free-piston. The compression ratio, which determines efficiency and combustion conditions as well as the amount of toxic emissions, can only be controlled by changing the pressure in the compression (bouncing) accumulator. However, this results in loss of energy and is very time consuming. Moreover, the need for four control surfaces and the requirement of an additional accumulator increase expense, and require additional space and reduce the efficiency of the free-piston engine.
Charge Mechanism
The utilization of exhaust gas energy increases the efficiency and reduces weight and size by increasing the specific power output, resulting in a smaller engine with less heat and friction losses.
In U.S. Pat. No. 5,261,797, there is disclosed a pressure wave charger (pulse pressure booster) which consists of a compressor, driven by an exhaust turbine or the crankshaft, and a booster, having a spring loaded booster piston, which is reciprocally mounted in a piston bore of a booster housing. The ingress of fresh air from the compressor to one chamber at the first end of the booster piston is controlled by a non-return valve. The egress to the combustion chamber is controlled by a rotary valve. A chamber at the second end of the booster piston, opposite to the first end, is in communication with the sump of the two-stroke combustion engine, controlled by a valve. A second chamber, being in communication with the exhaust port of the combustion engine, can be arranged at the second end of the booster piston, further increasing the pressure of the pressurized air from the compressor in the booster chamber.
Starting in the top-end position after the ignition, the combustion piston advances toward the bottom-end position, compressing the air in the sump while the compressor charges the first booster chamber with fresh, pressurized air. Near the bottom-end position of the combustion piston, the exhaust port and the valve to the sump of the engine open and provide pressure to their respective chambers at the second end of the booster piston in opposition to the compressed air. Pressure, sufficient to overcome the forces of the compressed air and a spring at the opposite side of the booster piston, will advance the piston toward the top-end position and increase the pressure of the compressed fresh air being forced through the intake port into the combustion chamber. Due to the spring force, the booster piston will return in its original position when the ports of the combustion engine are closed during the compression stroke.
The charge apparatus is complex and the overall efficiency low due to friction, leakage and larger amounts of compressed air, not participating on the combustion process.
Fuel Injection Apparatus
Known fuel injection systems provide fuel of increasingly higher pressure levels to a single, centrally or nearly centrally located fuel injector with one or several closely spaced nozzles to provide improved conditions for a more efficient combustion with reduced toxic emissions. The higher injection pressure provides an improved air-fuel mixture for a more efficient and cleaner combustion/lower toxic emissions, but the increased injection pressure consumes part of the gain in efficiency of the combustion process.
It is therefore, an object of the invention to provide a simplified hydraulic control mechanism for the free-piston, an efficient and robust extraction of energy from the exhaust gas and a simplified fuel injection apparatus which allows a more efficient and cleaner combustion.
Another object of the invention is to reduce known technical shortcomings of prior art free-piston engines or pumps, in particular, the limited ability to control the top-end and bottom-end positions of the piston in an inherently simple and efficient manner.
The present invention provides a novel hydraulic pressure-step force control mechanism for moving a piston or plunger of any apparatus, and for example, for moving the free-piston of an internal combustion engine, a pump, or the like, which hydraulic pressure-step force control mechanism results in a more accurate control of the top and bottom end positions of the piston. In addition, the invention provides a simple exhaust pressure wave charger. Moreover, the invention provides an improved fuel injection apparatus which introduces fuel into the combustion chamber under high pressure conditions and provides a more even distribution over the entire combustion chamber while preventing concentration of jets of atomized fuel within the combustion chamber, resulting in faster, more even combustion characterized by higher efficiency and reduced toxic emissions and soot.
In accordance with the invention there is provided a free-piston internal combustion pump-engine which includes a housing including.a piston bore and at least one free-piston, mounted in the piston bore for reciprocating movement between a bottom-end position and a top end position. The free-piston has a drive end and a combustion end. The drive end of the free-piston cooperates with the piston bore to define a first chamber and a second chamber. The combustion end of the free-piston cooperates with the piston bore to at least partially define a combustion chamber. A control system produces pressure control forces for moving the free-piston between the bottom-end position and the top-end position during a compression stroke, the control system supplying pressurized fluid to the piston bore at the drive end of the free-piston for applying a pressure control force to the drive end of the free-piston to move the free-piston toward the top-end position. The control system varies the supply of pressurized fluid to the piston bore to thereby vary the pressure control force applied to the free-piston at different times during the compression stroke. The free-piston is moved toward the bottom-end position by an expansion pressure within the combustion chamber during an expansion stroke, causing the pressurized fluid to be extracted from the piston bore as the free-piston is moved toward the bottom-end position during the expansion stroke.
In one embodiment of the invention, the combustion end of the free-piston has an outer combustion face and an inner bounce face, and the drive end has two control surfaces, an inner face and an outer face, acting in opposition to the combustion face, to control the top-end and bottom-end positions of the free-piston. At the beginning of the compression stroke, when starting the engine or pump, both hydraulic control surfaces are in fluid communication with a medium pressure accumulator, resulting in a small differential hydraulic piston force. A valve shift depressurizes one of the control surfaces, acting in opposition to the other control surface, increasing the hydraulic piston force in the middle portion of the stroke. In response to a further valve shift, the pressure supply is shifted from the medium pressure accumulator to a high pressure accumulator which results in sufficient force to advance the free-piston into its top-end position. During ignition, the increased pressure in the combustion chamber drives the free-piston toward its bottom-end position. During the expansion stroke, the sequence of valve actuation is reversed and delayed in time to allow for the extraction of the combustion energy. During the expansion stroke, the air in the bounce chamber at the opposite side of the combustion chamber is compressed. The timing of the fast-acting, two-way valves, which control the pressure stages at the hydraulic control piston, and therefore the end positions, is preferably determined by an electronic powertrain controller, considering the operating conditions (e.g., hydraulic and compression pressure, piston velocity, etc.) needed to control the free-piston engine-pump effectively. In accordance with another aspect of the invention, the exhaust pressure wave is transformed into an intake pressure wave, divided by a pressure wave separator, to charge the combustion chamber with pressurized fresh air. This provides higher power density of the engine and improves the scavenging and fuel mixture process. The remaining exhaust energy is extractable at the outlet of the pressure wave charger.
In one embodiment, the pressure wave charger comprises a charge piston having an exhaust end and an air-intake end, reciprocally mounted in a piston bore. The piston bore has an exhaust end chamber in communication with the exhaust port of the combustion engine and the charger exhaust (muffler) port. The charge exhaust port is opened at the end of the charger piston stroke. The piston bore has an intake chamber in communication with an air intake port of the combustion chamber and an air intake (air filter). The exhaust side is pressurized by the exhaust gas pressure wave from the combustion chamber, advancing the charge piston, against the forces of the compressing intake air and a bias structure, toward the bottom-end position of the charger piston, filling the combustion chamber with new compressed air. The bias structure returns the charger piston, with the support of the collapsing exhaust pressure wave, back into the top-end position during the compression stroke of the combustion engine, while the charger piston draws fresh air to the chamber at the air-intake end.
The free-piston engine or pump further includes a novel fuel injection apparatus which provides a finer atomization and a more equal distribution of fuel within the ignition chamber. This results in more uniform air/fuel ratio and combustion, reducing the amount of toxic emissions and soot, and reducing the ignition delay, thereby improving efficiency. In one preferred embodiment, the fuel injection apparatus comprises a novel fuel pump including a fuel injector mechanism and a novel nozzle structure which defines a fuel conduction channel and one or more nozzles. The fuel pump includes a fuel piston mounted in a bore of the fuel pump housing for reciprocating movement within the bore for increasing the pressure of the combustion fuel being injected into the combustion chamber. A second fuel pump can be added to provide uninterrupted fuel supply from the fuel injection pump.
In one preferred embodiment, the nozzle structure includes a fuel injection ring which defines one or more fuel conduction channels, for providing communication between the fuel injector and a plurality of micro-slots which define a plurality of nozzles at the inner, circumferential face of the fuel injection ring which is disposed to encompass the periphery of the ignition chamber. The fuel conduction channels and the micro-slots are preferably configured as depressions, arranged at an axial face of the fuel injection ring, the depressions being covered and sealed by a second section of the injection ring, allowing long and very narrow slots for increased atomization. A comparable function can be obtained by forming a thin shim-like layer on a substrate, incorporating the nozzle arrangement in form of a discontinuities formed in the layer, between the two parts of the fuel injection ring. The circumferential arrangement of slots/nozzles prevents the concentration of jets of atomized fuel and provides a more even distribution over the whole ignition chamber. The assembly of more than one multi channel/micro-slot arrangement allows for differently timed injection or different combustion ingredients.